galactic bulge

The spheroidal mass of stars that forms the central hub of spiral
and lenticular galaxies –
the yoke, if such galaxies are imagined to resemble fried eggs. The bulge
diminishes in size relative to the galactic
disk in the sequence of spirals Sa to Sd. Stars that populate the bulge
are normally old, Population II objects, dating
back to their galaxy's earliest period. Studying bulges can therefore tell
astronomers about how galaxies formed
and evolved.

According to one current theory, a spiral galaxy begins as a giant, roughly
spherical rotating mass of gas and dust, which gradually flattens out at
the edges to create the disk. The original spherical shape lives on in the
outermost region of a galaxy, known as the galactic
halo, and, to a lesser extent, in the bulge. However, this view is challenged
by observations of some bulges. The bulge of the Triangulum
Galaxy (M33), for example, contains young- and intermediate-age stars,
and has a star distribution that suggests the disk goes all the way to the
center. This raises questions about how M33 as a whole formed and what triggered
the birth of the relatively youthful stars in its bulge.